ATLANTA — Nearly twice as many US households are smoke-free compared to 20 years ago, reflecting an increased awareness of the health hazards from secondhand smoke, but too many people still are exposed, according to a federal study released Thursday.

In the early 1990s, 43 percent of US homes were smoke-free, a figure that rose to 83 percent in 2010-2011, according to the study, issued by the Centers for Disease Control and Prevention.

More than 90 percent of homes without a smoker and nearly half of those with at least one adult smoker had smoke-free rules, the study said.

“It’s a shift in social norms,” said Brian King, lead author of the CDC study. “People no longer see smoking around non-smokers as socially acceptable behavior.”

Still, he said, more progress is needed, particularly in homes where smokers live, as secondhand smoke from cigarettes kills an estimated 41,000 non-smokers annually.

“We know there is no safe level of secondhand smoke,” King said. “The ultimate goal is to not expose people to a known carcinogen.”

The increased number of smoke-free homes is attributable in part to the diminishing segment of Americans who smoke.

Some 18 percent of Americans were smokers in 2012, down from 42 percent in 1965, the CDC said.

With smoking bans increasingly common at bars, restaurants and in private workplaces, homes are the primary source of secondhand smoke for children and non-smokers, the CDC said.

Roughly half of US residents are now covered by laws that ban smoking in public places, the agency said.

Smoking bans are now expanding to apartments, public housing and even to cars with children inside, King said.

Among states, the percentage of smoke-free homes ranged from a low of 69.4 percent in Kentucky and West Virginia to 93.6% in Utah during 2010-2011, the CDC said.

The study did not ask respondents to specify whether they were referring to tobacco or marijuana smoke in the home, the CDC said.